1-[(4-Methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-(3-(R)-amino-piperidin-1-yl) xanthine, whose international nonproprietary name is Linagliptin [CAS number: 668270-12-0], has the following chemical structure of formula I.

U.S. Pat. No. 7,407,955 (US '955) discloses Linagliptin, related compounds, and their pharmaceutical compositions. Further, it describes a process for the preparation of Linagliptin wherein tert-butyloxy carbonyl (Boc) protected Linagliptin is deprotected using 5-6 M isopropanolic hydrochloric acid followed by purification using chromatography. The process disclosed in US '955 is schematically represented in scheme-I.

U.S. Pat. No. 7,820,815 (“US '815) discloses a process for preparation of Linagliptin wherein it is prepared by deprotecting 1-[(4-methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-(3-(R)-phthalimidopiperidin-1-yl)-xanthine of formula IIIa in presence of ethanolamine. The 1-[(4-methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-(3-(R)phthalimidopiperidin-1-yl)-xanthine is prepared by condensing 1-[(4-methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-bromo xanthine of formula III with (R)-3-phthalimidopiperidine of formula IIa. The process disclosed in US '815 is schematically represented in scheme-II.

The prior art processes as disclosed in US '955 and US '815 involve protection-deprotection method, leading to an increase in the manufacturing cycle time, decrease in the product yield and further requires purification by chromatography that is not desirable for commercial-scale manufacturing. In addition, the prior art processes leads to formation of bromo-butene impurity (i.e. 1-[(4-Methyl-quinazolin-2-yl)methyl]-3-methyl-7-(3-bromobut-2-ene-1-yl)-8-(3-(R)-amino-piperidin-1-yl) xanthine of formula Ib.

It is therefore essential to develop simplified and improved process for preparation of pure Linagliptin that alleviates the deficits of prior art processes. Thus, the present application provides an improved process for preparation of pure Linagliptin and pharmaceutically acceptable salts thereof.